Structure of glycogen

This page discusses the structure of glycogen and is one of a series that discuss common errors in current textbooks of biochemistry.

All of the pages in this series are in urgent need of updating. The biochemical principles have not changed, of course, but textbooks have: some of those that were current when I first prepared these pages in 2000 have appeared in new editions, and others have ceased to be widely used. New books have appeared that are not discussed. Unfortunately I do not have easy access to any of the commonly used textbooks, as I work in a research (not teaching) environment in a country where English is not the everyday working language. I could buy them, of course, but that would represent rather a large investment for the sake of a few web pages.

Accordingly I should be grateful if someone would collaborate with me in the revision. If you have access to all of the textbooks published in English in the past ten years (say 1996 or later) that are commonly used for teaching biochemistry, and if you would like to help, please contact me at acornish@ibsm.cnrs-mrs.fr.

The actual structure

The structure of glycogen is essentially as shown in the following drawing, except that in the complete glycogen molecule there are 12 layers whereas for simplicity only 5 are shown in the drawing:

The essential features of this general structure are that each B-chain has two branch points, and all chains have the same length, as the experiments showed that the material is distributed at 50% between A- and B-chains.

In addition it is worth noting the molecule of glycogenin, a protein that acts as a primer, at the centre of the structure.

Despite the fact that this structure was worked out by Whelan’s group nearly thirty years ago, and is accepted by all workers in the glycogen field, i.e. there is no current controversy about it (see, for example, the review by Calder), virtually all textbooks show the completely obsolete structure suggested by Meyer and Bernfeld in 1940, and do not mention the protein component. This structure is distinguishable from the correct structure by a more haphazard appearance, without a clear distinction between A and B chains, with variable chain lengths and with more than or less than two branch points per B-chain. In addition, of course, the glycogenin molecule at the core is omitted, as this was not suspected in 1940.

What the textbooks say

All of the textbooks examined either give the incorrect structure or show the structure so vaguely that it is difficult to guess whether it is based on the correct one or not. If they mention glycogenin at all they do so in a different context from the one in which the structure is given. The only textbook that I know of (see below) that presents the information correctly was not among those considered in relation to the other errors discussed.

Despite showing the obsolete structure, Campbell refers in her text (pp. 419–420) to some of the results from modelling the structure (results that refer, however, to the real structure, not the one she shows):

It has been shown by mathematical modeling that the structure of glycogen is optimized for its ability to store and deliver energy quickly and for the longest amount of time possible. The key to this optimization is the average length of the branches (13 residues). If the average chain length were much greater or much shorter, glycogen would not be as efficient a vehicle for energy storage and release on demand. Experimental results support the conclusions reached from the mathematical modeling.

Elongation stops when the synthase is no longer in contact with glycogenin, which forms the core of the particle. The synthase-glycogenin interaction limits the size of glycogen granules. We see here a simple and elegant molecular device for setting the size of a biological structure.

However, this simple and elegant molecular device appears to be pure fantasy. It is not explained how a small 38 kDa protein buried deep inside a 1000 kDa polymer can remain in contact with an enzyme working on the surface.

I thank Enrique Meléndez-Hevia for informing me that the correct structure is shown in the chapter by R. K. Murray, D. K. Granner, P. A. Mayes & V. W. Rodwell in Harper’s Biochemistry (24th edn., Prentice-Hall International, London, 1996, p. 143). As this book was not among those I had easy access to when making the original survey it is not at present included in these pages. However, I shall try to include it when I have more information about the other questions considered.

Textbook checklist

Abeles, Frey and Jencks	Structure of whole molecule not indicated; no glycogenin	p. 435–437
Campbell	Meyer and Bernfeld (1940) structure; no glycogenin	pp. 419–420
Garrett and Grisham	Structure of whole molecule not indicated; no glycogenin	p. 678
Horton et al.	Meyer and Bernfeld (1940) structure; glycogenin mentioned later (p. 385)	p. 232
Lehninger, Nelson and Cox	Meyer and Bernfeld (1940) structure; glycogenin mentioned later (p. 614)	p. 309
McKee and McKee	Meyer and Bernfeld (1940) structure; glycogenin mentioned later (p. 183)	p. 168
Mathews, van Holde and Ahern	Meyer and Bernfeld (1940) structure; glycogenin mentioned later (p. 573)	p. 472
Stryer	Unclear, but appears to be Meyer and Bernfeld (1940) structure; glycogenin assigned a fantastic function	p. 19
Voet and Voet	Meyer and Bernfeld (1940) structure; glycogenin mentioned later (p. 492)	p. 485
Zubay	Meyer and Bernfeld (1940) structure; no glycogenin	p. 9

References

P. C. Calder (1991) Glycogen structure and biogenesis Int. J. Biochem. 23, 1335-1352

Z. Gunja-Smith, J. J. Marshall, C. Mercier, E. E. Smith and W. J. Whelan (1971) A revision of the Meyer–Bernfeld model of glycogen and amylopectin, FEBS Lett. 12, 101–104